The object of the invention is a method for the holographic recording of data. In the method a hologram containing the date is recorded in a waveguide layer as an interference between an object beam and a reference beam. The object beam is essentially perpendicular to the plane of the hologram, while the reference beam is coupled in the waveguide. There is also proposed an apparatus for performing the method. The apparatus comprises a data storage medium with a waveguide holographic storage layer, and an optical system for writing and reading the holograms. The optical system comprises means for producing an object beam and a reference beam, and imaging the object beam and a reference beam on the storage medium.
Storage systems realised with tapes stand out from other data storage systems regarding their immense storage capacity. Such systems were used to realise the storage of data in the order of Terabytes. This large storage capacity is achieved partly by the storage density, and partly by the length of the storage tapes. The relative space requirements of tapes are small, because they may be wound up into a very small volume. Their disadvantage is the relatively large random access time.
The random access time may be decreased, or the capacity may be increased with the same random access time, if the data storage is not only done in the plane of the storage medium, but also in depth (so-called 3D or three-dimensional storage). Optical data storage offers several possibilities for 3D storage. One possible way is the solution used in multi-layered CD-s, or the DVD. In this case the data storage planes are spaced apart by some tens of xcexcm-s. The applied optical system has a large numerical aperture, with a depth resolution of approx. 1 xcexcm, and a precise focus servo system allows the selective readout of the layers placed beneath each other.
Another solution is known from the area of holographic data storage. In this case the data are stored as thick holograms (Bragg holograms). Here the xe2x80x9cdepth addressingxe2x80x9d, i.e. the separation of the holograms recorded into the same physical volume, may be achieved with the Bragg conditions. This is known as angle-, wavelength-, displacement- etc. multiplexing. In the experimental holographic storage systems in the laboratories primarily crystals are used as storage medium (Fe doped LiNbO3). This finds only limited applications, due to considerations of manufacturing technology, and may not be used at all for tape storage systems. For this purpose only a polymer type material is feasible.
Polymer based materials are normally produced in large quantities, relatively easily, and are easily fixed on a substrate. Known optical storage materials are the so-called side-chain polymers, and their use in holograms is also known. Another important property of these polymers is that no post-exposure treatment is necessary, e.g. no subsequent development, thermal or electrical fixing process. This is a very important issue in all practical data storage technology.
It has been shown experimentally that so-called side-chain polymers are excellently suitable for optical data storage purposes. Thin polarisation holograms may be recorded in side-chain polymers with a theoretical 100% efficiency. However, in order to record Bragg holograms that are suitable for spatial (three-dimensional) storage, at least 25-50 xcexcm thick holographic storage material is necessary. Polymer materials with such thickness undergo substantial deformation (as a result of the change in temperature, mechanical impacts, humidity, etc.). The deformation of the holographic storage layer will cause the deformation of the lattice in the hologram, and this will in turn lead to a decrease in diffraction efficiency. As the layer thickness increases, and the lattice deformation increases therewith, beyond a certain threshold the thick hologram will deteriorate, and finally it will be unreadable. On the contrary, thin holograms are much less sensitive to the deformation of the holographic lattice.
Therefore, it is an object of the present invention to provide a structure, which allows the recording of multiple holograms within the same unit area of the data storage medium, and at least partly eliminates the problems above. With the invention a data storage structure is suggested, which allows in-depth data storage in case of thin polymer storage materials. The suggested solution combines the advantages of thin holograms (insensitivity to lattice deformation) with the advantages of thick holograms (three-dimensional, in-depth storage, large data density).
According to the method of the invention, a hologram containing the data is recorded in or in the vicinity of a waveguide layer as an interference between an object beam-and a reference beam, where the object beams is essentially perpendicular to the plane of the hologram, while the reference beam is coupled into the waveguide. According to the invention, the holograms are formed in a layer structure containing multiple waveguide layers, and coupling means are provided for selectively coupling the reference beam into the single waveguide layers of the layer structure.
In a preferred realisation of the method a grating is used as the coupling means. Advantageously, gratings with different pitch and/or profile are used in the different waveguide layers, and the reference beam is projected at different incidence angles on the coupling means, i.e. the gratings.
Alternatively, gratings with different orientation may be used in the different layers. In this case the reference beam is projected from different directions onto the coupling means, where the projections of the different directions projected on the plane of the data storage medium are also different.
The invention also relates to an arrangement or system for holographic recording of data. The system comprises a data storage medium with a waveguide holographic storage layer, and an optical system for writing and reading the holograms. The optical system comprises means for producing an object beam and a reference beam, and imaging the object beam and a reference beam on the storage medium. According to the invention, the arrangement comprises multiple waveguide holographic storage layers in the storage medium, and further comprises means for selectively coupling the reference beam into the single waveguide layers of the layer structure.
In a preferred embodiment, the optical system comprises means for projecting the reference beam onto the coupling means from different incidence angles. In this case it is particularly advantageous if the coupling means comprises gratings with different pitch and/or profile in the different layers.
In a further preferred embodiment, the optical system comprises means for projecting the reference beam onto the coupling means from different directions, where the projections of these different directions in the plane of the data storage medium are also different from each other, i.e. the components of the directions falling in the plane of the data storage medium point in different directions in that plane. For this arrangement it is suggested that the coupling means comprises gratings with different orientation in the different layers.
In a further particularly preferred embodiment, it is foreseen to use a tape as the data storage medium. In order to facilitate the smooth readout of the running tape, in a particularly preferred embodiment the optical system is positioned in a rotating cylinder, where the cylinder is guiding the tape. In order to provide a continuous readout of the data on the tape, in an advantageous embodiment there is provided multiple optical systems within the cylinder.
The invention also concerns the waveguide structures used in the optical system of the invention.